Spin valve element and method of manufacturing same

ABSTRACT

A spin valve element may include a plurality of magnetic element groups. Each magnetic element group may be formed, at least in part, by a plurality of magnetic elements being connected in parallel. Each magnetic element may include an intermediate layer and a pair of ferromagnetic layers sandwiching the intermediate layer. The plurality of magnetic element groups may be connected together in series or in parallel. The plurality of magnetic elements may be configured to undergo a microwave oscillation and are placed in proximity sufficient that oscillation signals are configured to be generated with the magnetic elements mutually synchronized. The proximity may include a range equal to a wavelength of the microwave oscillation.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

The present application is a continuation and claims priority under 35U.S.C. § 120 to U.S. patent application Ser. No. 12/739,741, filed Jul.28, 2010, now U.S. Pat. No. 9,318,248, which claims priority under 35U.S.C. § 371 to International Application No. PCT/JP2008/065407, filedAug. 28, 2008, which claims priority to Japanese Patent Application Ser.No. 2007-277399, filed on Oct. 25, 2007. The entire contents of theabove-mentioned prior-filed applications are hereby expresslyincorporated herein by reference.

TECHNICAL FIELD

This invention relates to spin valve elements, and to a manufacturingmethod of such elements. More specifically, this invention relates tospin valve elements employing the tunneling magneto-resistance (TMR)effect or the giant magneto-resistance (GMR) effect, and to a method ofdriving such elements.

BACKGROUND ART

With recent advances in nanoelectronics, products are being developedwhich apply physical phenomena unique to magnetic materials with minutesizes. Of these, there have been particularly rapid advances in thefield of spin electronics, which utilize the spin of free electrons inmagnetic materials.

In the field of spin electronics, spin valve elements utilizing thetunneling magneto-resistance (TMR) effect occurring in a layeredstructure of a ferromagnetic layer, an insulating layer, and aferromagnetic layer in order, or utilizing the giant magneto-resistance(GMR) effect occurring in a layered structure of a ferromagnetic layer,nonmagnetic layer (conducting layer), and a ferromagnetic layer inorder, are currently regarded as having the greatest possibility ofapplication.

FIG. 5 and FIG. 6 are cross-sectional views showing the configuration ofspin valve elements of the prior art. Of these, FIG. 5 shows the basicconstituent portions of a spin valve element utilizing TMR. This elementhas a configuration in which an insulating layer 24, and a ferromagneticlayer 23 (fixed layer) and ferromagnetic layer 25 (free layer)sandwiching the insulating layer, are formed on a substrate 5; to thisare further added, as necessary, electrode layers 21, 27, anantiferromagnetic layer (pinning layer) 22, a capping layer 26, andsimilar. The direction of the magnetization of the fixed layer 23 isfixed by magnetic coupling with the antiferromagnetic layer 22 andsimilar. When electrons are passed from the fixed layer 23 toward thefree layer 25 in this element, a torque acts to cause the magnetizationof the free layer 25 to be aligned parallel to the direction of themagnetization of the fixed layer 23. Conversely, when electrons arepassed from the free layer 25 toward the fixed layer 23, a torque actson the magnetization of the free layer 25 so as to be antiparallel tothe direction of the magnetization of the fixed layer 23. Consequentlydepending on the direction of the current in the free layer 25, thedirection of magnetization of the free layer 25 can be controlled. Thisphenomenon of magnetization inversion by electron spin is called spintransfer magnetization reversal. For reasons described below, inconventional structures the size in in-plane directions of the magneticlayers in such an elements must be kept to very small sizes(approximately 150 nm or less), and expensive equipment such as electronbeam exposure equipment is used.

In order to suppress the exchange coupling due to the leakage magneticfield from the film edge portions of the ferromagnetic layers 23 (fixedlayer) and 25 (free layer) sandwiching the insulating layer 24,generally the portion on the side above the insulating layer 24 is madesufficiently smaller than that on the substrate side, and an insulatingfilm 30 is formed on the periphery thereof. A number of methods may beused to form these structures; for example, after forming the layeredfilm from the substrate to the electrode 27, applying a negative resistand performing exposure using a photolithography method, ion milling canbe performed to expose the upper portion of the insulating layer 24, andthereafter an insulating layer 30 can be formed by covering with SiO₂ orsimilar, and after liftoff the electrode 27 to be used for wiring can beformed.

FIG. 6 shows the basic constituent portions of a spin valve elementutilizing GMR. The difference between the element utilizing TMR of FIG.5 and the element utilizing GMR is the replacement of the insulatinglayer 24 with a nonmagnetic layer 51; otherwise the configuration isbasically the same.

Magnetic random access memory (MRAM) is attracting the most attention asan application of these technologies, and is drawing interest as areplacement for conventional DRAM (dynamic random access memory) andSRAM (synchronous DRAM).

Further, it is known that when an electric current and an externalmagnetic field are simultaneously applied to these spin valve elements,microwave oscillation occurs (see, for example, S. I. Kiselev, et al,“Microwave oscillations of a nanomagnet driven by a spin-polarizedcurrent”, Nature, Vol. 425, p. 380 (2003)). For example with respect tothe current direction, suppose that a current is passed such that thetorque acts on the magnetization of the free layer 25 so as to becomeantiparallel to the direction of the magnetization of the fixed layer23, and suppose that an external magnetic field causes a torque to acton the magnetization of the free layer 25 so as to become parallel tothe direction of the magnetization of the fixed layer 23. In this case,under conditions in which the two torques are counterbalanced,high-frequency oscillation in the microwave region can be induced.

In addition, it has been reported that when two elements are formedadjacently and when currents and external magnetic fields appropriate tothese are applied, the oscillation frequencies and phases of the twobecome coincident, the frequency width is decreased, and microwaveoutput at this time is also increased (see, for example, Kaka, et al,“Mutual phase-locking of microwave spin torque nano-oscillators”,Nature, Vol. 437, p. 389 (2005); F. B. Mancoff, et al, “Phase-locking indouble-point-contact spin-transfer devices”, Nature, Vol. 437, p. 393(2005); J. Grollier, et al, “Synchronization of spin-transfer oscillatordriven by stimulated microwave currents”, Physical Review B73, p. 060409(2006)). This phenomenon is called a phase locking phenomenon, and themechanism, though not yet be clarified, is inferred to arise frominteraction between the high-frequency magnetic fields occurring in eachof the elements; this phenomenon is attracting attention as means ofincreased output.

In numerous reports, the oscillation output of the above microwaveoscillator element stops at approximately 0.16 μW for TMR and atapproximately 10 pW for GMR, which are very low levels for practicalapplication. The simplest means to increase output is to increase theelement area, but this is difficult for the following reason. That is,in spin valve elements, in order to facilitate coherent rotation ofspins necessary for spin-transfer magnetization inversion, the magneticfilms must comprise a single magnetic domain. For example, in order toobtain a single magnetic domain in the magnetic film the periphery ofwhich is circumscribed on the left and right by the insulating film 30in FIG. 5 and FIG. 6, the size circumscribed by the insulating film 30on the left and right must be made small. In this way, the size of theelement is required to be at most a size in which domain walls do notexist; although varying with material and shape, this size isapproximately 150 nm. The size of a single conventional spin valveelement cannot be made larger than this dimension.

Because there is an upper limit to the size of one spin valve element,in order to obtain a large output, numerous minute elements must beintegrated. As means of integration, photolithography techniques aremost widely used and have high precision; but in order to fabricatemagnetic members with microminiature sizes (approximately 150 nm orless), investment in electron beam exposure and similar expensiveequipment is necessary, so that there is the problem of highmanufacturing costs.

Further, in “High-regularity metal nano-hole array based on anodicoxidized alumina”, control of the size, pitch and depth of porous holeswhen manufacturing a porous alumina film from an aluminum film throughmanipulation of external conditions is disclosed (see H. Masuda,“High-regularity metal nano-hole array based on anodic oxidizedalumina”, Kotai Butsuri, Vol. 31, No. 5, p. 493, 1996). And, in“Nanoscopic templates using self-assembled cylindrical diblockco-polymers for patterned media”, an invention is disclosed which isintended for applications in so-called bit-patterned media of hard disks(see X. M. Yang, et al, “Nanoscopic templates using self-assembledcylindrical diblock co-polymers for patterned media”, J. Vac. Sci.Technol. B, Vol. 22, p. 3331 (2004)).

However, when numerous elements such as described above are connected inparallel, the overall impedance of the integrated element declines withthe number of elements, that is, with increasing total area of theelements. However, in general in high-frequency circuits it is necessaryto match impedances in order to suppress transmission losses. In themicrowave region, generally input and output impedances are set to be50Ω. Even when there are such input/output impedance settings, theoscillation output can be increased even when elements are connected inparallel as described above, but measures must be taken with respect tothe overall electrical resistance, which declines as the number ofelements increases. Further, there is the problem that if magneticelements are simply connected in parallel, synchronized oscillation maynot occur between these elements.

Further, in addition to parallel connection of spin valve elements, theoscillation output can also be increased through series connections. Inthe case of series connections, as the number of elements increases theoverall electrical resistance increases. In order to use spin valveelements in series connections, a method in which photolithography isused to form numerous planar separate elements which are connected byseparate wiring, and a method in which a separate substrate is used forwiring to connect elements, are conceivable. However, in methods inwhich numerous separate planar elements are formed by electron beamexposure or other means in widespread use, there is the same problem asin the prior art of the need to invest in expensive equipment, and in amethod in which elements formed on a separate substrate are connected bywiring, there are such practical problems as an increase in the numberof processes and limitations on the number of connected elements arisingfrom limitations on the element dimensions.

In light of these circumstances, a low-cost method is sought foradjustment the element impedance and suppressing transmission losses ofthe oscillation microwaves.

Moreover, when spin valve elements are manufactured using porous film,there is an upper limit to the sizes of satisfactory porous films whichcan be obtained, and formation of spin valve elements of arbitrary areausing porous film is difficult. Consequently even when a lower impedanceis desired, broadening the spin valve element area to adjust theimpedance is difficult.

DISCLOSURE OF THE INVENTION

In light of the above circumstances, this invention has as an object theprovision, at low cost, of a large-output microwave oscillation element,in which numerous spin valve elements are integrated, and moreoverelement impedance is adjusted and transmission losses are suppressed.

In considering the fact that in order to synchronize microwaveoscillation elements it is necessary that a plurality of microwaveoscillation elements (spin valve elements) be arranged within a certainappropriate range and be driven simultaneously, the inventors of thisapplication studied configurations with matching to prescribedimpedances. As a result, it was discovered that even in cases in whichseparate spin valve elements oscillate in mutual synchronization, it iseffective to connect a plurality of microwave oscillation elements inseries or in parallel in order to perform impedance matching.

In this invention a spin valve element is provided, comprising aplurality of parallel magnetic element groups, formed by connecting inparallel a plurality of magnetic elements including at least threelayers of an intermediate layer formed by an insulating member or anonmagnetic member and a pair of ferromagnetic layers sandwiching theintermediate layer, the parallel magnetic element groups being connectedtogether in series or in parallel. Further, in this invention a spinvalve element is provided, comprising a plurality of series magneticelement groups, formed by connecting in series a plurality of magneticelements of at least three layers of an intermediate layer formed by aninsulating member or a nonmagnetic member and a pair of ferromagneticlayers sandwiching the intermediate layer, the series magnetic elementgroups being connected together in parallel.

The above inventions were devised from the discovery by the inventorsthat when a plurality of magnetic elements to be are caused to oscillatein synchronization, and moreover impedance matching is to be performedappropriately, these can be attained by means of a spin valve element(magnetic element) in which magnetic elements are connected in acombination of series and parallel connections. In a mode of thisinvention, magnetic elements can be placed in such proximity that mutualsynchronization and generation of oscillation signals is possible. Inthis case, synchronized oscillation actually occurs in a plurality ofspin valve elements, and factors affecting the impedance at this timeare the interaction between spin valve elements based on the microwavesand other electromagnetic waves of the oscillation; however, the typescannot always be identified. Spin movement depends on the effectivemagnetic field and spin injection current acting on the spin, andoscillation synchronization phenomena are induced by the AC componentsof these. Of these, the interaction between series-connected spin valveelements may possibly be due to the AC component of the current, anddoes not depend on the distance between elements, but the interactionbetween parallel-connected spin valve elements is inferred to be due tothe magnetic field arising from spin rotation, and the inventors of thisapplication has confirmed that the mutual distance between spin valveelements plays an important role. In this case, it is preferable, forthe purposes of synchronized oscillation and impedance matching, thatthe spin valve elements be arranged within a range approximately equalto the wavelength of the oscillation microwaves.

By means of any of the embodiments of the invention, impedance matchingcan be achieved while inducing the synchronized oscillation of aplurality of spin valve elements which undergo microwave or otherhigh-frequency oscillation. Moreover, by means of any of the embodimentsof this invention, a spin valve element can be manufactured using aninexpensive manufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view showing the configuration ofthe spin valve element of a first embodiment of the invention;

FIG. 2 is an enlarged cross-sectional view of principal portions,showing the configuration of the spin valve element of the firstembodiment of the invention;

FIG. 3 is a horizontal cross-sectional view showing the configuration ofthe spin valve element of the first embodiment of the invention;

FIG. 4 is a vertical cross-sectional view showing the configuration ofthe spin valve element of a second embodiment of the invention;

FIG. 5 is a cross-sectional view showing the configuration of a spinvalve element of the prior art; and

FIG. 6 is a cross-sectional view showing the configuration of a spinvalve element of the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

In this invention, a plurality of spin valve elements are positioned inproximity, and are electrically connected together. As the method offormation of spin valve elements, conventional electron beam exposure orother techniques can be used. Below, embodiments of the invention areexplained referring to the drawings.

First Embodiment

FIG. 1 is a vertical cross-sectional view showing the configuration ofthe spin valve element of a first embodiment of the invention, and FIG.2 is an enlarged cross-sectional view of principal portions, showing theconfiguration of the spin valve element of the first embodiment. In thisembodiment, as shown in the figures, a plurality of spin valve layers 20are layered in a plurality of minute holes 12 formed in the insulatingmember 11 in a porous insulating layer 10 so as to be series-connected,and these can be connected in parallel by an electrode 31 and anelectrode layer 21. Here, layered portions are shown in the figures, butin this invention a configuration is also possible in which a singlespin valve layer 20 in the minute holes 12 is placed in each minute hole12, and layering is not performed. The following description takes as anexample a case in which the spin valve layers 20 have a GMR structure,but similar functions can be obtained in this embodiment even for a TMRstructure.

As shown in FIG. 2, a spin valve layer 20 in this embodiment comprises anonmagnetic layer 51 and a pair of ferromagnetic layers 23 (fixed layer)and 25 (free layer) which sandwich the nonmagnetic layer; as necessary,an antiferromagnetic layer (pinning layer), capping layer (neithershown), and similar may be layered onto for example one of theferromagnetic layers 23. Within the minute holes 12 are layered magneticelements comprising spin valve layers 20 having this structure andintermediate electrodes 29 as shown in FIG. 2; in the interior of eachminute hole 12 is a group of series-connected magnetic elements (aseries magnetic element group).

Electrodes 31, 21 are placed on the upper and lower faces of the seriesmagnetic element groups in each minute hole 12; in this way, each of theseries magnetic elements in each minute hole 12 are electricallyconnected with the outside of the minute hole 12. The electrodes 31, 21connect a plurality of magnetic element groups in parallel.

Representative examples of the materials forming spin valve elements inthe first embodiment include silicon substrate or glass substrate as thesubstrate 5; Ta, Pt, Cu, or Au as the electrode layers 21, 29, 31; IrMnor PtMn as the antiferromagnetic layer (not shown); Co, CoFe, or CoFeBas the ferromagnetic layer 23 (fixed layer); MgO or an Al oxide as theinsulating layer 24; Cu as the nonmagnetic layer 51; Co, CoFe, CoFeB, orNiFe as the ferromagnetic layer 25 (free layer); and Cu or Pd as thecapping layer (not shown). However, materials are not limited to these.By using the same material in the ferromagnetic layer 23 (fixed layer)and in the ferromagnetic layer 25 (free layer), and making the filmthickness of the former greater than the film thickness of the latter, aspin valve function can be achieved through the difference incoercivities. In manufacturing, after layering each of the abovematerials, annealing treatment in a magnetic field is effective toadjust the crystallinity of each layer and the magnetic anisotropy ofthe fixed layer. Where necessary, the ferromagnetic layer 23 (fixedlayer) and ferromagnetic layer 25 (free layer) can also be anantiferromagnetic coupled film of for example CoFeB/Ru/CoFeB or similar.Notation in which a plurality of materials are delimited by a slash (/)indicates a multilayer film in which layers of the materials are layeredin that order.

Methods of forming the porous insulating layer 10 include for exampletreatment to perform anodic oxidation of an aluminum film, treatment toform microstructures in a resin film by self-organization, and treatmentto transfer a microstructure by a nanoimprinting technique. Any of thesemethods can be used to form minute holes with uniform shape by means ofan inexpensive process. In particular, if nanoimprinting is used, minuteholes can be formed with a high aspect ratio (ratio of the hole depth tothe hole diameter), which is preferable for forming a layered structurewith many layers. Nanoimprinting also has the advantage of facilitatingcontrol over shape.

FIG. 3 is a horizontal cross-sectional view of the porous insulatinglayer 10 showing this example. In general in a spin valve element, themagnetic films are often formed into an elliptical shape for the purposeof controlling the direction of the in-plane anisotropy of the magneticfilms, but a shape such as that shown in FIG. 3 can also be formedeasily.

Differing from this, in anodic oxidation treatment of aluminum film theminute hole shapes are circular, and it is difficult to control thein-plane anisotropy direction of the magnetic films. However, when usingperpendicular magnetization types in which the crystal anisotropy isaligned in the direction perpendicular to the film plane, the limits tosuch shape controllability do not pose a problem, and this invention canbe implemented. Further, in microstructure formation treatment byself-organization of a resin as well, the minute hole shapes arenormally circular, which is advantageous for perpendicular magnetizationtypes in which the crystal anisotropy is aligned in the directionperpendicular to the film plane. In resin self-organization, minuteholes are obtained with an aspect ratio higher than that fornanoimprinting techniques, and the method is suitable for obtaininglayered structures with many layers.

As a method of manufacturing a porous insulating layer 10 usingnanoimprinting, for example a thermoplastic resin such as polymethylmethacrylate or similar is applied to a substrate, after which heatingis performed at a temperature higher than the glass transition point ofthe thermoplastic resin to soften the thermoplastic resin, and then astamper is pressed to transfer the relief pattern of the stamper. Bycooling the substrate, a porous insulating layer 10 having a prescribedminute hole structure can be obtained on the substrate. The stampermaterial is generally silicon, quartz, silicon carbide, tantalum, orsimilar; for use in this invention, silicon, which can bemicro-machined, is particularly appropriate. In order to improve therelease properties of the stamper and the polymer layer, it ispreferable that a fluoride polymer and surfactant be applied to thestamper surface. As the material of the porous insulating layer 10, anultraviolet ray hardening resin can be used; after pressing the stamper,photohardening can be performed, to eliminate the processes of heatingand cooling the substrate.

As a polymer for use in this nanoimprinting method, polymethylmethacrylate, polystyrene, polycarbonate, and other thermoplasticresins, 1,6-hexane diol diacrylate, bis hydroxy ethyl-bis-phenolA-dimethylacrylate, and similar may be used; however, materials whichcan be used as porous films in this invention are not limited to these.

By means of this nanoimprinting method, if within the range in which thestamper machining precision is obtained, the relief pattern can beaccurately transferred. Compared with machining of separate elements byelectron beam lithography or similar, manufacturing costs are far moreinexpensive. There may be cases in which there are upper limits to therange over which the stamper machining precision can be maintained orupper limits to the size of a stamper with high machining precision, orcases in which cost is extremely high at large sizes. Hence the rangeover which appropriate minute holes 12 are obtained in the porous film10 is necessarily limited. In this embodiment, individual spin valveelements (magnetic elements) are manufactured having an area extendingover the range in which such appropriate minute holes 12 are obtained.And, spin valve elements manufactured in island shapes are connectedtogether as in FIG. 4 such that the impedance is the desired value. Bythis means, the trade-off between cost and performance with respect tothe maximum size and the relief pattern accuracy of a stamper can beresolved while keeping impedance matching.

The technique of subjecting an aluminum film to anodic oxidationtreatment to obtain a porous alumina film is itself a technique alreadyin use, but the technique for fabricating the nanometer-order minuteholes required has not yet been practically applied. In methods usinganodic oxidation treatment, in essence the external conditions aremanipulated to control the size, pitch and depth of the porous holes(see, for example, H. Masuda, “High-regularity metal nano-hole arraybased on anodic oxidized alumina”, Kotai Butsuri, Vol. 31, No. 5, p.493, 1996). By means of this method, nanometer-order minute holes can beformed densely in two dimensions. In particular, a porous alumina layerhas high heat resistance, and so is sufficiently durable with respect tothe annealing treatment necessary for a spin valve structure, explainedbelow, and is therefore preferable with respect to the processes of thisinvention.

As the method of manufacturing the porous alumina layer 10, first analuminum film is formed by a sputtering method or another method. Asnecessary heat treatment can be performed in an inert gas or in vacuumat 400 to 500° C. to coarsen the crystal grains and decrease crystalgrain boundaries, so that the order of arrangement of minute holes inthe porous alumina layer can be improved. Thereafter, an aqueoussolution of H₃PO₄ or H₂SO₄, or similar, is used to perform electrolyticpolishing, to flatten the surface. Anodic oxidation treatment isperformed using for example oxalic acid as the electrolytic solution,and by using a constant voltage of approximately 30 to 60 V as theformation voltage, minute holes are formed, regularly arranged and withhigh density. The regularity of arrangement of these minute holesadvances with the passage of time in the anodic oxidation treatment, sothat by performing anodic oxidation treatment over a long period oftime, highly regular, densely arranged minute holes can be formed.Further, the intervals between minute holes formed by anodic oxidationtreatment can be controlled through the applied voltage, and isapproximately 2.8 nm/V. That is, when 40 V is applied, the intervals areapproximately 112 nm. The ratio of intervals to diameters of minuteholes depends on the electrolytic solution and the treatmenttemperature, but can be adjusted within the range of approximately 1.5to 5. As one example, the diameter per unit voltage when using oxalicacid as the electrolytic solution was approximately 4.9 nm/V, so thatwhen 40 V is applied, the diameter is approximately 23 nm.

In a porous alumina layer 10 obtained as described above, numerousminute holes are formed in a regular arrangement, and the holes areformed perpendicularly to the surface of the porous alumina layer 10,but the bottom portions are closed, forming cylindrical spaces. Whenminute holes with elliptical cross-sections are formed, in order topenetrate these holes, after anodic oxidation treatment, immersiontreatment in H₃PO₄ or similar must be performed.

In recent years, there have been efforts to develop technology tomanufacture porous insulating layers 10 utilizing phenomena in which amicrostructure is formed through resin self-organization, aiming atapplications in so-called bit patterned media of hard disks (see, forexample, X. M. Yang, et al, “Nanoscopic templates using self-assembledcylindrical diblock co-polymers for patterned media”, J. Vac. Sci.Technol. B, Vol. 22, p. 3331 (2004)). In this technology, in essence asolution of two types of non-miscible polymers is applied onto asubstrate, and after heat treatment to induce phase separation of thepolymers, one of the polymers is removed by chemical means to obtain aminute hole structure. Normally, this method can be used to obtainminute holes several tens of nanometers in diameter, at a pitch ofseveral tens of nanometers.

As a method of manufacturing a porous insulating layer 10 utilizingself-organization of a resin, for example a polystyrene-methylmethacrylate (PS-PMMA) copolymer is dissolved in toluene or anothersolvent, and this is applied onto a substrate by spin-coating or anothermethod. Here, copolymer components of approximately PS:PMMA=70:30 areappropriate for obtaining a porous structure. The spin-coatingconditions and solvent concentration can be adjusted as appropriatedepending on the film thickness of the porous insulating layer 10 whichis desired; for example, to obtain a thickness of 40 to 50 nm, aspin-coating revolution rate of 1800 to 2400 rpm and a solid componentconcentration of 1 to 3% by weight are desirable. Then, upon annealingin vacuum for several hours at 170° C., the polystyrene (PS) andpolymethyl methacrylate (PMMA) undergo phase separation. One of thepolymers (in the above material, PMMA) is selectively removed, to obtaina porous structure. In the case of the above composition, afterirradiation with ultraviolet light to degrade the PMMA, washing withglacial acetic acid and water is performed to remove the PMMA, so thatholes of diameter 20 nm at a pitch of approximately 40 nm can beobtained.

In order to form minute holes with a high aspect ratio perpendicularlyto the substrate, the substrate may be treated with a self-organizingfilm or similar in advance, to adjust the surface energy of thesubstrate. As the polymers used in this method, polystyrene (PS),polymethyl methacrylate (PMMA), polyisoprene, polylactide, and similarare used, but polymers which may be used are not limited to these.

The intervals between the minute holes formed by the above method can becontrolled mainly through the copolymer component ratio. The diametersof the minute holes are determined by the ratio of surface energies ofthe polymer materials at the time of phase separation, and arecontrolled by the copolymer materials, the solvent, the annealingtemperature, and other factors.

A porous insulating layer 10 formed by nanoimprinting or by resinself-organization comprises a polymer material, and in general has poorheat resistance. For this reason, after having formed the magneticmultilayer structure and electrodes in the minute holes, it is effectiveto remove the porous insulating layer 10, cover the remaining minutecolumnar structures of magnetic multilayer film with SiO₂ or anotherinorganic insulating layer, and then polish the surface to expose theelectrodes, to form a porous insulating layer 10 with high heatresistance. This means is made necessary by the process of annealing ofthe spin valve elements in particular. As the means of covering withSiO₂ or another inorganic insulating material, CVD, or application ofTEOS (tetraethoxysilane, Si(OC₂H₅)₄) followed by heat treatment forconversion into SiO₂, or other means can be used.

The intervals between minute holes of the porous layer can be chosenarbitrarily; the inventors have discovered, with respect to the effectof increasing output through phase locking between spin valve elementsin particular, that this originates in the interaction due to theelectromagnetic fields of microwaves from oscillation of each of thespin valve elements. Hence in order to obtain this effect, a distancebetween the spin valve elements approximately equal to the wavelength ofthe microwaves equivalent thereto is sufficient. That is, if themicrowaves have frequency 20 GHz, for example, then the wavelength inthe atmosphere is approximately 15 mm, and so an increase in the outputof spin valve elements within this range due to phase locking can beexpected. Moreover, even at distances equal to or greater than this, ifthe elements are connected by electrical wiring, a similar effect can beobtained.

Second Embodiment

FIG. 4 shows one example of the spin valve element of a secondembodiment of the invention. In the spin valve element of the secondembodiment, integrated spin valve elements, obtained for example by theabove method, are separated into island shapes approximately severalmicrometers on a side on a substrate, within a range which overall isapproximately 100 μm. And, these spin valve elements are connected inparallel or in series on the substrate, and the entirety is used as aspin valve element. As explained above, the dimensions of separate spinvalve elements (magnetic elements) must be approximately 150 nm or less,and the wiring between these elements requires expensive investments inelectron beam exposure or other equipment. The second embodiment of theinvention adopts a configuration in which a spin valve element group, inwhich are integrated several hundred to several thousand elements, areformed at once, and a plurality of element groups are provided and thesegroups are wired. By adopting this structure, the individual elements ofelement groups can be manufactured without using lithography. By thismeans, wiring and entire element groups can be formed using inexpensivevisible light exposure, and a configuration is possible in whichadequate performance is obtained through patterning using visible light.Individual spin valve element groups separated into island shapes mayhave a simple parallel structure in which single-layer spin valves areformed in minute holes, or can have a series-parallel structure in whichlayered spin valve elements are formed in minute holes, as in the firstembodiment. When single-layer spin valve elements are formed, eachisland-shape spin valve element group is a parallel magnetic elementgroup. At this time, individual island-shape magnetic elements having aparallel element group can be further connected in parallel. In the casefor example of a single magnetic element having the same area as the sumof all the magnetic elements, if within this area there is ashort-circuited portion at even a single place, the entirety isshort-circuited; but by separating into a number of groups andconnecting in parallel only islands which have been inspected withoutproblem, the pass rate can be improved.

The impedance of individual spin valve elements depends on the filmthickness and the element area, but in the case of TMR is approximately50Ω to 1.5Ω, and in the case of GMR is approximately 5 to 50Ω, so thatin order to make the impedance of the entire element 50Ω, it isappropriate that the ratio (number of series connections):(number ofparallel connections) be 1:100 to 1:30,000 for TMR, and be 1:1 to 10:1for GMR.

In this way, using a porous insulating layer, a spin valve layer can beformed in the minute holes thereof to obtain an inexpensive spin valveelement. And, by layering a plurality of spin valve layers in the minuteholes, series-structure spin valve elements can be obtained; and inaddition, by arranging these in a parallel structure, a series-parallelstructure spin valve element can be obtained. Or, by grouping aplurality of spin valve elements formed in parallel on a substrate, andby series-connecting these at once, the lithograph precision requiredcan be lowered and manufacturing costs can be reduced. By means of theseconfigurations, the number of parallel connections and the number ofseries connections of spin valve layers can be arbitrarily selected, sothat the impedance of the spin valve element can easily be adjusted.

In this way, numerous spin valve elements can be integrated, andmoreover the element impedance can be adjusted and transmission lossessuppressed, to provide a high-output microwave oscillator element at lowcost.

Below, a practical example of each of the above embodiments isexplained.

Practical Example 1

Using the structure of the First Embodiment, a TMR layer was firstfabricated by the following procedure. Following each of the materials,the film thickness of the layer is indicated in parentheses. A Cu (80nm) thin film was formed on the silicon substrate 5 by a sputteringmethod as the electrode layer 21. Then, Co₇₀Fe₃₀ (20 nm) as theferromagnetic layer 23, MgO (0.6 nm) as the insulating layer 24,Co₄₀Fe₄₀B₂₀ (2 nm) as the ferromagnetic layer 25, Cu (2 nm) as thecapping layer (not shown), and Pt (10 nm) as the intermediate electrodelayer 29, were layered in order, to obtain one spin valve layer 20. And,by repeating this process, a total of five spin valve layers werelayered.

Next, negative resist was applied onto the uppermost face of the spinvalve layers, and elliptical regions were irradiated with an electronbeam so as to form regions arranged in a honeycomb pattern of ellipseswith major axis of 120 nm×minor axis of 60 nm, and with hole centers atdistances of 320 nm. By this means resist remained in the ellipsepatterns, and a negative resist pattern arrangement was obtained. And,ion milling was performed using this resist pattern to remove all fivelayers of the spin valve layers immediately below the regions withoutresist. Then, in order to form the insulating member 11 (FIG. 1, FIG.2), a CVD method was used to form an SiO₂ film. After removing theresist on the spin valve element by lift-off, a negative resist wasagain applied, photolithography was used to obtain a plurality of 6 μmdiameter resist patterns, and by using these to remove regions in whichthere was no resist, 6 μm diameter islands were obtained. Approximately176 spin valve layer structures were formed within one 6 μm diameterisland. After separating the resist on the 6 μm diameter islands,sputtering was used to layer Cu and form an upper electrode 31, andannealing was performed at 350° C. in a magnetic field of approximately4 kOe to fabricate a sample of Practical Example 1. The electricalresistance of the spin valve layers 20 in this configuration wasapproximately 1.77 kΩ per layer, or a total of approximately 8.85 kΩ forfive layers; and as a result of parallel-connecting the approximately176 minute holes in a 6 μm diameter island with the electrode 21 andelectrode 27, the electrical resistance for an entire island wasapproximately 50Ω.

Practical Example 2

A Cu (80 nm) thin film was formed by sputtering as an electrode layer 21on a silicon substrate 5. Then, a toluene solution of polymethylmethacrylate (solid component concentration 3%) was applied byspin-coating and dried to obtain a polymethyl methacrylate thin film(thickness 120 nm). Then, this was heated at approximately 120° C., andby pressing a silicon stamper, a porous film structure, in which apattern of minute elliptical 120 nm×60 nm holes were arranged in ahoneycomb structure with a distance between hole centers of 400 nm (FIG.3), was transferred and fabricated. Then, photoresist was applied to theporous film, and exposure performed to form a plurality of holes ofdiameter 6 μm. Within the 6 μm diameter islands were formedapproximately 112 minute holes. Then, ion etching was used to remove thebottom portions and form penetrating holes.

Then, sputtering was used to layer, in order in the minute holes 12,Co₇₀Fe₃₀ (20 nm) as the ferromagnetic layer 23, MgO (0.8 nm) as theinsulating layer 24, Co₄₀Fe₄₀B₂₀ (2 nm) as the ferromagnetic layer 25,and Cu (2 nm) as the capping layer (not shown), to obtain one spin valvelayer 20, after which Pt (10 nm) was further layered as an intermediateelectrode layer 29. By repeating this process, a total of three spinvalve layers were layered, The photoresist was then separated andremoved.

Then, Cu was layered by sputtering as the upper electrode layer 31,filling the minute holes 12, after which the remaining polymer wasremoved by oxygen plasma treatment, to form spin valve layered columnstructures. Further, after injecting TEOS (tetraethoxysilane) in thespace between layered column structures, heat treatment at 400° C. wasperformed to convert the material to SiO₂, to pack the space of thelayered column structures with SiO₂ and cover the layered columnstructures. Then, the surface was polished to expose the Cu of the upperelectrode layer 31. By means of this process, the material comprised bythe porous insulating layer 10 was converted from a polymer into SiO₂having heat resistance. After extending the upper electrode 31 to theupper portion of the porous insulating layer 10, annealing was performedat 350° C. in a magnetic field of approximately 4 kOe, to obtain asample of Practical Example 2.

As a result, the number of minute holes in a 6 μm diameter island ofthis configuration was approximately 112, the electrical resistance ofthe spin valve layer 20 was approximately 13.0 kΩ per layer and wasapproximately 39.0 kΩ for three layers, and the electrical resistancefor an entire island was approximately 348Ω. In this case, byparallel-connecting seven islands, approximately 50Ω was obtained as theoverall impedance of the element. Wiring between islands was formed byapplication of a negative resist and an ordinary photolithography methodusing visible light for exposure processing, followed by sputtering toform Al wiring lines.

Practical Example 3

Apart from a number of modifications, a sample of Practical Example 3was manufactured similarly to that of Practical Example 2. Themodifications were as follows. The solid component concentration of thetoluene solution of polymethyl methacrylate was 5% in spin coating. Thefilm thickness at this time was 1200 nm. By this means, a porous layer10 of polymethyl methacrylate was formed. Here, the elliptical minuteholes had a honeycomb structure pattern, with a distance of 200 nmbetween hole centers. And, division was performed by lift off, so as toobtain islands with a diameter of 6 μm. By this means, the number ofminute holes in each island was approximately 450. And, within theminute holes 12 formed in the porous layer 10, spin valve layers wereformed as follows.

First, electroplating was used to layer in order, in minute holes 12,Ni₈₀Fe₂₀ (20 nm) as the ferromagnetic layer 23, Cu (6 nm) as thenonmagnetic layer 51, Ni₈₀Fe₂₀ (2 nm) as the ferromagnetic layer 25, andCu (2 nm) as the capping layer 26, then the 30 spin valve layers to forma spin valve layer utilizing GMR, after which Pt (10 nm) was formed asthe intermediate electrode layer 29. The annealing conditions were 250°C. in a 4 kOe magnetic field, but otherwise, the sample of PracticalExample 3 was fabricated similarly to that of Practical example 2. Thelayered thickness from the substrate to the upper portion of the porousinsulating layer 10 was approximately 1200 nm.

The electrical resistance for a spin valve layer 20 with thisconfiguration was 22.9Ω per layer, and the total for 30 spin valvelayers was 687Ω; the electrical resistance for an entire island wasapproximately 1.53Ω. And, by series-connecting 33 islands, an impedanceof approximately 50Ω was obtained for the entire element.

Practical Example 4

The sample of Practical Example 4 was fabricated by a methodsubstantially similar to that of Practical Example 1. Similarly toPractical Example 1, sputtering was used to form Cu (80 nm) thin film asthe electrode layer 21 on a silicon substrate 5. And, the sample ofPractical Example 4 was obtained by a method similar to that ofPractical Example 1, except that Co₇₀Fe₃₀ (20 nm) as the ferromagneticlayer 23, an MgO layer (0.6 nm) as the insulating layer 24, NiFe (4.5nm) as the ferromagnetic layer 25, Cu (2 nm) as the capping layer 26,and Pt (10 nm) as the intermediate electrode layer 29 were layered inorder, and by repeating this process, a total of three spin valve layerswere layered; the dimensions of the elliptical fine hole patterns was30×50 nm, and the distances between hole centers in the honeycombstructure was 140 nm; the dimensions of the divided islands were 2 μm indiameter, and a total of eight of these islands were formed on the samesubstrate; and the arrangement was in four rows and two columns, at a100 μm pitch. Approximately 100 minute holes were formed in a 2 μmdiameter island, and for a total of eight islands, 800 layered spinvalve elements were formed.

The electrical resistance for a spin valve layer 20 with thisconfiguration was 13.5 kΩ per layer, and the total for three layers wasapproximately 40.5 kΩ; the electrical resistance for an entire islandwas approximately 405Ω. And, by parallel-connecting eight islands, animpedance of approximately 50Ω was obtained for the entire element.

Practical Example 5

The sample of Practical Example 5 was fabricated by a methodsubstantially similar to that of Practical Example 1. After treating asilicon substrate 5 with an OTS (octadecyltrichlorosilane)self-organization film, a polystyrene-methyl methacrylate (PS-PMMA)copolymer thin film was applied by spin coating. That is, a toluenesolution of a polystyrene-methyl methacrylate copolymer (manufactured byPolymer Science Inc., PS:PMMA=70:30) (solid component concentration 5%by weight) was applied at a spin coating revolution rate of 900 rpm, toobtain a thin film of thickness 4800 nm. Then, by performing annealingfor three hours at 170° C. in vacuum, the polystyrene (PS) and thepolymethyl methacrylate (PMMA) phases were separated. Then irradiationwith ultraviolet light was performed to degrade the polymethylmethacrylate (PMMA), after which washing with glacial acetic acid andwater were performed for selective removal, to obtain a porous structurein which minute holes were arranged regularly in a honeycomb structure.The minute holes were perpendicular to the substrate and cylindrical inshape, and had a diameter of approximately 20 nm and pitch ofapproximately 40 nm.

Then, electroplating was used to layer in order, in minute holes 12, Ru(5 nm)/Co₇₀Fe₃₀ (20 nm) as the ferromagnetic layer 23, Cu (6 nm) as thenonmagnetic layer 51, NiFe (4.5 nm) as the ferromagnetic layer 25, Cu (2nm) as the capping layer 26, and Pt (10 nm) as the intermediateelectrode layer 29, to obtain a single spin valve layer 20. By repeatingthis process, a total of 100 spin valve layers were layered. Then, afterusing sputtering to layer Cu as the upper electrode layer 31 and fillingthe minute holes 12, means similar to that of Practical Example 2 wasused to convert the material comprised by the porous insulating layer 10from a polymer to SiO₂ having heat resistance. After using sputtering tolayer Cu and extend the upper electrode 31 to the upper portion of theporous insulating layer 10, annealing was performed at 250° C. in amagnetic field of approximately 4 kOe, to fabricate the sample ofPractical Example 5. The layered thickness from the substrate to theupper portion of the porous insulating layer 10 was approximately 4800nm.

The electrical resistance for a spin valve layer 20 with thisconfiguration was 310Ω per layer, and the total for 100 spin valvelayers was approximately 31 kΩ. Upon forming islands with area such thatthe diameter was 2 μm, 1250 minute holes existed in one island, and theimpedance for an island was approximately 24.8 kΩ. And, byseries-connecting two of these islands, an impedance of approximately50Ω was obtained for the entire element.

By subjecting the samples of Practical Examples 1 through 5 to a 1 T DCmagnetic field in the direction parallel to the magnetic field of thefixed layer, and passing a DC current in the direction such thatelectrons flow from the free layer into the fixed layer, microwaveoscillation was obtained. Measurement results are shown in Table 1.

TABLE 1 Number Element of spin Driving Total Individual Total resistancevalve voltage input element output Frequency (Ω) layers (V) (W) output(W) (W) (GHz) Practical 50 8.8E+02 5 5.0E−01 1.5E−06 1.3E−03 23.4Example 1 Practical 50 2.4E+03 15 4.7E+00 1.4E−06 3.3E−03 17.8 Example 2Practical 50 4.5E+05 0.27 1.6E+00 9.0E−11 4.0E−05 26.3 Example 3Practical 51 2.4E+03 3 1.9E−01 1.8E−06 4.3E−03 13.6 Example 4 Practical50 2.5E+05 0.9 7.1E−02 1.4E−10 3.5E−05 14.7 Example 5

As shown in FIG. 1, by means of this invention, from several thousandsto several hundred thousand spin valve elements could be combined, withthe overall impedance matched to approximately 50Ω, to fabricate amicrowave oscillator element with high output (from 35 μW to 1.8 mW).This is a large improvement over conventional single-element output(0.16 μW when using TMR, approximately 10 pW when using GMR). Further,as the average output for each spin valve layer in each minute hole inthis invention, 1.5 μW, 1.4 μW, and 1.8 μW respectively were obtained inPractical Examples 1, 2 and 4 using TMR, and 90 pW were obtained inPractical Example 3 using GMR, so that large improvements were obtainedfor single elements as well. The reason for this is inferred to arisefrom the phase locking phenomenon observed when a plurality of elementsare formed in proximity, as explained above.

In the above, embodiments of the invention have been described, but theinvention is not limited to the embodiments described, and variousmodifications, alteration, and combinations are possible, based on thetechnical concepts of the invention. By means of the invention, numerousspin valve elements can be integrated, and the element impedance can beadjusted to suppress transmission loss, to provide at low cost ahigh-output microwave oscillator element.

The invention claimed is:
 1. A spin valve element, comprising: aplurality of magnetic element groups, wherein each magnetic elementgroup is formed, at least in part, by a plurality of magnetic elementsbeing connected in parallel, and wherein each magnetic element includesan intermediate layer and a pair of ferromagnetic layers sandwiching theintermediate layer; wherein: the plurality of magnetic element groupsare connected together in series or in parallel; the plurality ofmagnetic elements are configured to undergo a microwave oscillation andare placed in proximity sufficient that oscillation signals areconfigured to be generated with the magnetic elements mutuallysynchronized; and the proximity is in a range equal to a wavelength ofthe microwave oscillation.
 2. The spin valve element of claim 1, furthercomprising a porous layer having a plurality of minute holes, whereinthe plurality of magnetic elements are placed within the plurality ofminute holes, wherein the plurality of magnetic elements are connectedin parallel outside of the plurality of minute holes respectively, andwherein the plurality of magnetic elements form a parallel magneticelement group.
 3. A spin valve element manufacturing method ofmanufacturing the spin valve element of claim 2, comprising a step offorming the porous layer by nanoimprinting.
 4. A spin valve elementmanufacturing method of manufacturing the spin valve element of claim 2,comprising a step of forming the porous layer by either: a step ofperforming anodic oxidation of an aluminum thin film; or a step ofcausing self-organization of a resin film.
 5. The spin valve element ofclaim 2, wherein the porous layer is formed by nanoimprinting.
 6. Thespin valve element of claim 1, wherein the intermediate layer is aninsulating layer or an electrically-conductive nonmagnetic layer.
 7. Thespin valve element of claim 1, wherein: each of the pair offerromagnetic layers has a predetermined thickness; and each of the pairof ferromagnetic layers is the same material.
 8. The spin valve elementof claim 7, wherein the pair of ferromagnetic layers comprises a fixedlayer and a free layer, and wherein the predetermined thickness of thefixed layer is greater than the predetermined thickness of the freelayer.
 9. The spin valve element of claim 1, wherein the plurality ofmagnetic element groups are formed on the same substrate.
 10. A spinvalve element, comprising: a plurality of magnetic element groups,wherein each magnetic element group is formed, at least in part, by aplurality of magnetic elements being connected in series, and whereineach magnetic element includes an intermediate layer and a pair offerromagnetic layers sandwiching the intermediate layer; wherein: theplurality of magnetic element groups are connected together in parallel;the plurality of magnetic elements are configured to undergo a microwaveoscillation and are placed in proximity sufficient that oscillationsignals are configured to be generated with the magnetic elementsmutually synchronized; and the proximity is in a range equal to awavelength of the microwave oscillation.
 11. The spin valve element ofclaim 10, further comprising a porous layer having a plurality of minuteholes, wherein the plurality of magnetic elements are placed within theplurality of minute holes respectively, and wherein the plurality ofmagnetic elements are connected in parallel outside of the plurality ofminute holes.
 12. A spin valve element group, comprising a plurality ofthe spin valve elements of claim 11, wherein the spin valve elements areconnected in series.
 13. A spin valve element manufacturing method ofmanufacturing the spin valve element of claim 11, comprising a step offorming the porous layer by nanoimprinting.
 14. A spin valve elementmanufacturing method of manufacturing the spin valve element of claim11, comprising a step of forming the porous layer by either: a step ofperforming anodic oxidation of an aluminum thin film; or a step ofcausing self-organization of a resin film.
 15. The spin valve element ofclaim 11, wherein the porous layer is formed by nanoimprinting.
 16. Thespin valve element of claim 10, wherein the intermediate layer is aninsulating layer or an electrically-conductive nonmagnetic layer. 17.The spin valve element of claim 10, wherein: each of the pair offerromagnetic layers has a predetermined thickness; and each of the pairof ferromagnetic layers is the same material.
 18. The spin valve elementof claim 17, wherein the pair of ferromagnetic layers comprises a fixedlayer and a free layer, and wherein the predetermined thickness of thefixed layer is greater than the predetermined thickness of the freelayer.